Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METAL DETECTOR COIL CONFIGURATION TO ELIMINATE ORIENTATION EFFECT
Background
Metal detectors are used to detect metals and/or metal contaminants in product
streams. Metal
detectors detect metal as they pass through a plane of detection defined by
the orientation of detection
coils within the system. There are limitations in the ability of various metal
detector systems to detect
metals based on the orientation of the metal object as it passes through the
plane of detection. What is
needed is a solution that eliminates or reduces the orientation effect that
prior art metal detectors are
prone to.
Summary
What is presented is a system for metal detection comprising a single aperture
that further
comprises two or more sets of detection coils that surround the perimeter of
the aperture. A flow path
of materials passes through the aperture. Each set of detection coils
comprises a transmitter coil and
two receiver coils, with the transmitter coil located between the two receiver
coils. Each set of detection
coils is at a different angle relative to the flow path. In some embodiments
of metal detection systems,
the aperture could comprise three sets of detection coils that surround the
perimeter of the aperture.
In some embodiments, one set of detection coils is at an angle of 45 degrees
relative to the flow
path. In some embodiments, one set of detection coils is at an angle of 135
degrees relative to the flow
path. In various embodiments, the flow path is one of a conveyor belt, a
liquid line, or free-falling
material. The aperture may be circular or a polygon. In various embodiments,
the detection coils
operate in the range of 1kHz to 1MHz. Each set of detection coils may be
operated in a different
detection frequency or in the same detection frequency.
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Those skilled in the art will realize that this invention is capable of
embodiments that are
different from those shown and that details of the apparatus and methods can
be changed in various
manners without departing from the scope of this invention. Accordingly, the
drawings and descriptions
are to be regarded as including such equivalent embodiments as do not depart
from the spirit and scope
of this invention.
Brief Description of Drawings
For a more complete understanding and appreciation of this invention, and its
many
advantages, reference will be made to the following detailed description taken
in conjunction with the
accompanying drawings.
FIG. 1 shows a simplified schematic of a prior art metal detection system;
FIG. 2 shows a simplified schematic of the principle of operation of metal
detection systems;
FIG. 3 depicts a metal contaminant passing through a detection plane in
various configurations;
FIG. 4 depicts a simplified schematic of a prior art metal detection system
comprising two
separate metal detection systems with two apertures at different angles to the
material flow path;
FIG. 5 is a simplified schematic of an embodiment of the metal detection
system having two sets
of detection coils;
FIG. 6 is a simplified schematic of another embodiment of the metal detection
system having
two sets of detection coils;
FIG. 7 depicts a metal contaminant passing through a detection plane in
various configurations;
FIG. 8 depicts a metal contaminant passing through a detection plane in
various configurations;
and
FIG. 9 is a simplified schematic of another embodiment of the metal detection
system having
three sets of detection coils.
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Detailed Description
Referring to the drawings, some of the reference numerals are used to
designate the same or
corresponding parts through several of the embodiments and figures shown and
described.
Corresponding parts are denoted in different embodiments with the addition of
lowercase letters.
Variations of corresponding parts in form or function that are depicted in the
figures are described. It
will be understood that variations in the embodiments can generally be
interchanged without deviating
from the invention.
FIG. 1 shows a simplified schematic of a prior art metal detection system 10.
The system
comprises an aperture 12 through which a flow path of materials passes
through. A single set of
detection coils 14 surrounds the perimeter of the aperture 12. The set of
detection coils 14 comprise a
transmitter coil 16 (also called an oscillator coil) and two receiver coils
18, with the transmitter coil 16
located between the two receiver coils 18.
FIG. 2 illustrates the basic operating principle of the type of metal detector
to which this
disclosure applies. A single voltage is driven through the transmitter coil 16
by an oscillator (not shown).
The two receiver coils 18 are located on a common axis with the transmitter
coil 16 and are coupled into
the electromagnetic field of the transmitter coil 18 in such a manner that the
system is in balance and
the induced voltages in the two receiver coils 16 cancel. Material to be
screened that passes through the
aperture 12 creates a distortion in the electromagnetic field if there is any
metal passing along with the
material in the flow path through the aperture 12. This distortion results in
a difference in the induced
voltages in the two secondary coils. The voltage difference is amplified,
digitized, and filtered to extract
detection information that is used to decide whether the signal represents
metal or the user's product.
If the signal represents the user's product, it is ignored. If it represents
metal that exceeds a pre-set
sensitivity level, the detector generates a detection signal that initiates
reject and/or alarm actions.
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Referring to FIG. 3, for long thin metal contaminants 22 (referred to herein
as a "wire," but
could be a needle, staple, metal shaving, etc.), metal detectors typically
have more difficulty detecting
the contaminant in one orientation than in other orientations. For example, if
a wire 22 passes through
the metal detector with its long dimension parallel to the detection plane 20
of the detector coil, it may
produce a significantly larger signal than if the same wire 22 passed through
in an orientation
perpendicular to the detection plane 20 of the detector coil. This is referred
to as the "orientation
effect," and it occurs because the detection signal is related to the area of
eddy current loops which are
developed in the wire 22. In FIG. 3 (a)-(d) shows a cylindrical contaminant
wire 22 passing through the
detection plane 20 at several angles, and FIG. 3 (a1)-(di) shows the
corresponding cross-sectional areas
of the wire 22 intersecting the detection plane 20.
If a metal contaminant 22 type develops eddy current loops parallel to the
detection plane 20 of
the detector coils, they produce their largest signal in orientation shown in
FIG. 3(a), because that gives
the largest area for the eddy current loops as shown in FIG 3(ai). The
contaminant 22 orientation shown
in FIG. 3 (d) has the smallest cross-sectional area shown in FIG. 3(di), thus
producing the smallest signal.
Some metal contaminant 22 types are the opposite, developing their signal
based on cross-
sectional area perpendicular to the detection plane 20. For these metal types,
FIG. 3 (d) is the best case
for detection sensitivity.
Metal detector sensitivity is usually specified based on detecting a metal
sphere, which has no
orientation effect. For a sphere, the cross-sectional area parallel to the
plane is equal to the area
perpendicular to the plane, and the cross-section is always the same
regardless of rotation of the
sphere. The smallest sphere (of a given metal type) which is detectable by a
given metal detector is
called the "rated metal sphere" for that detector and metal type.
A wire can arrive at the detector in any random orientation, so assuming the
worst case, and the
sensitivity for wires must be specified based on the diameter of the wire,
regardless of length. Wires
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with a diameter greater than or equal to the rated metal sphere diameter will
typically be detectable
even in the worst orientation. A smaller-diameter wire, even if relatively
large compared to the rated
metal sphere size, has a risk of passing through without being detected.
Most metal detector installations simply accept (or ignore) this risk. Where
orientation effect
was considered an unacceptable risk, FIG 4 shows an example of a prior art
metal detector 10
installations using two separate metal detectors 10, installed on the same
conveyor line 24 on which a
will material stream pass through. The metal detectors 10 are oriented at
different angles relative to the
conveyor line 24. This has several disadvantages: it requires two complete
metal detectors 10; it
requires a longer conveyor line 24 system; in order to be able to be placed at
an angle, the metal
detectors 10 must be much wider than the conveyor line 24; it assumes that the
wire does not change
orientation while traveling between the two metal detectors 10; and with two
metal detectors 10 it still
does not completely eliminate orientation effect for metal types which have a
worst case parallel to the
coil, as explained later.
FIG. 5 shows a simplified schematic of metal detector 10a disclosed herein
that addresses some
of the limitations of the prior art systems. The system comprises an aperture
12a through which a flow
path of materials passes through. Two sets of detection coils 14a surrounds
the perimeter of the
aperture 12a. Each set of detection coils 14a comprises a transmitter coil 16a
and two receiver coils 18a,
with the transmitter coil 16a located between the two receiver coils 18a. In
this embodiment the
detector coils 14a are crossed on the top and bottom of the aperture 12a. The
aperture 12a is shown as
a having a rectangular opening in the figures, but it could be any shape
required by the application such
as any other polygon or a circle. The flow path of material that passes
through the aperture 12a may be
a conveyor belt, a liquid line, or free-falling material.
The system presented herein comprises two sets of metal detection coils 14a
within one metal
detector 10a housing. Each set of detection coils 14a comprises a transmitter
coil 16a and two receiver
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coils 18a, all parallel to each other. However, each set of detection coils
14a is at a different angle
relative to the direction of material travel through the aperture 12a. The
detection coils 14a operate in
a detection frequency in the range of 1kHz to 1MHz.The separate sets of
detection coils 14a are
operated preferably at different detection frequencies, but it is possible
they could be operated at the
same detection frequency.
In some embodiments of powering the transmitter coils 16a, the separate
transmitter coils 16a
interfere with each other if there is mutual inductance coupling each
transmitter coil 16a, so the
transmitter coils 16a must be separated by a great enough angle to reduce this
mutual inductance to an
acceptable level. A 90 angle between each detection coil 14a system reduces
the mutual inductance to
the minimum possible. Other angles are possible to reduce interference from
each other, but a large
angle (ideally 90 ) is still desirable, for the maximum reduction of
orientation effect. Embodiments
where one set of detection coils is at an angle of 45 relative to the flow
path have found to be effective.
Embodiments where one set of detection coils is at an angle of 135 relative
to the flow path have also
found to be effective. However, using a large angle has a disadvantage of
requiring a larger metal
detector (i.e., a longer tunnel in the direction of product travel).
FIG. 6 shows another embodiment of the metal detection system 10b having two
sets of
detection coils 14b in which the detector coils 14b are crossed on the sides
of the aperture.
As illustrated in FIG. 7, in the systems presented, metal contaminants 22 are
simultaneously
subjected to detection planes 20 from more than one angle. So, if the
contaminant 22 is in the worst
orientation for detection at one detection plane 20, it can be in a more
favorable orientation for the
other detection plane 20. For metal types which have the worst-case detection
perpendicular to the
detection plane 20, two sets of detection planes 20 are enough. The two sets
of detections coils form
intersecting detection planes 20 as shown in FIG. 7 (a)-(b) and (c)-(d). A
wire contaminant 22 in any
orientation cannot be perpendicular to both detection planes 20.
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Metal contaminant 22 types which have the worst-case detection parallel to the
detection plane
20 will require three sets of detection planes 20 to eliminate the orientation
effect. Two sets detection
planes 20 having orientations as shown in FIG. 7 would greatly reduce the
problem. However, FIG. 8 (a)-
(b) shows there would still be one orientation at which a contaminant wire 22
could be parallel to two
detection planes 20, so a third detection plane 20 as shown in FIG. 8 (c) is
needed.
FIG. 9 shows an embodiment of the metal detection system 10c having three sets
of detection
coils 14c for three different intersecting angles of detection planes.
Embodiments with four sets of
detection coils are possible but would have limited utility for eliminating
orientation effects since all
orientations are able to be handled by three sets of detection coils.
This invention has been described with reference to several preferred
embodiments. Many
modifications and alterations will occur to others upon reading and
understanding the preceding
specification. It is intended that the invention be construed as including all
such alterations and
modifications in so far as they come within the scope of the appended claims
or the equivalents of these
claims.
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